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In which situations can a Dual-Aircraft Platform (atmospheric satellite) remain nearly geostationary?
The Dual-Aircraft platform described by William Engblom of Embry-Riddle Aeronautical University on this NASA feature page and in the first half of the video of the 2016 NIAC Symposium uses the wind sheer (difference in horizontal wind velocities) between two altitudes to allow a well-programmed pair of tethered solar gliders/planes to remain in a general area (e.g. 150 km radius) above the Earth.
The analogy is kite sailing, except in this case both fluids are air.
In the simulations presented, the aircraft have propellors which can generate forward thrust using electrical power, or reverse thrust when the motor is used as a generator (analogous to regenerative braking on electric vehicles).
I'm having problems understanding the point of this system - can it do something that a single solar-electric atmospheric satellite can not do? Can the horizontal wind sheer provide some special advantage for "hovering" near a certain location on the ground? Is a small (say 10 degree) difference in wind direction at the two altitudes enough to remain roughly geostationary - or is a much larger difference needed?
The second questioner (a bit after 00:20:00) seems to pose a simple but interesting question about tacking, and the third questioner's mention of conservation is also interesting.
Below: screen shots from the 2016 NIAC Symposium

With a number of efforts to provide atmospheric satellite coverage for future internet and other services - would the creation of an atmospheric-satellite tag here be premature? I wanted to attach a solar-power tag but right now only solar-impuse is available. see for example.

Under the guidance here, I'd say that your question validates a new tag: the criteria isn't "Will it be used in future?" the criteria is "Am I asking a question that does not fit under another tag?". I believe the answer to that is "Yes". I don't believe you had the privilege to create it, so I've done so

Almost any situation
The only time you wouldn't be able to stay (near) Geostationary would be if the wind velocity is higher than your own maximum powered velocity, at all altitudes you are able to operate.
In any other situation, you simply fly in something resembling a circle over the area you with to loiter, or fly directly into the wind (zig-zagging if the wind is slower than your minimum airspeed). Which you choose will depend on your minimum/maximum airspeed, the windspeed, and how "stationary" you need to remain with reference to a point on the ground.
Then again, the above applies to almost any solar powered aircraft, so how about the specifics of this concept?
What are the potential advantages?
Winds at altitude are not always (or even usually) in the same direction, or at the same velocity. We can take advantage of this in some scenarios:
If you have, for example, a fast-moving stream of air near a slower moving body of air, or (even better) a body moving in roughly the opposite direction, you can fly slowly "upwind" in the slow moving stream (which doesn't use much energy, relatively speaking) then "surf" down the fast stream, harvesting energy as you go - you then use that gained energy to fly back into position, through the slow moving body.
If you get an even more ideal scenario, the slow moving body is in the opposite direction to the fast moving body, you can simply "float" around in the currents, harvesting energy most of the time.
The important part to note is that zig zagging (akin to tacking in sailing) - regular tacking requires you to change direction regularly to avoid going off course... with two aircraft tethered, however, they can "tack" away from each other without changing direction. Essentially, they can angle away from each other and vary the angle of incidence to remain relatively stationary within the wind: if the wind speed increases, you fly more "into" the wind, if it decreases, you fly more perpendicular to the wind (and therefore more "away" from your partner aircraft).
This combination of two aircraft tack-surfing (totally not a technical word, I definitely just made it up) mean you can "surf" the wind even when the windspeed  is lower than your own stall speed, without having to tack as a single aircraft would. Tacking has a couple of disadvantages - it's more involved, and it means you are constantly travelling laterally across the wind, while two aircraft surfing together can stay in almost exactly the same place, merely by angling more or less into the wind. They can also, by coordinating power between themselves, move laterally along the "face" of the wind
In short, you have much more fine-grained control over your position, in more conditions

Wow, this helps a lot, and gives me a lot to think about! In the "tack-surfing" mode, any idea if the tether is let out and reeled back in each time to maintain tension, or it stays out and they somehow maneuver to maintain tension?

@uhoh in this specific example I have no idea - I don't know exactly how their project is intended to work. Theoretically, either could work: manoeuvring to maintain tension shouldn't actually be too difficult, you just "slip" into the wind by dropping your upwind wing, exactly like you would when landing

OK thanks very much - I'm trying to wrap my head around this and every little bit helps. There is some talk in the video about active control of tether deployed length and this made me think of that. I wonder when this will show up in popular simulator software!


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Can we submit and sign the 8710 in IACRA after the check ride?
I work at a 142 school and was told we can submit and esign the 8710 after the check ride.  The digital esignature shows a date time stamp 9-hours after the beginning of the oral and flight exam. I have always had everyone submit and esign the 8710 before any event and completed it after the event with the result.
Thoughts?  Is there any regulation or FAA guidance one way or the other?

Welcome to aviation.SE! Can you clarify what "esign" means here? I'm not sure if you're asking if you can do the checkride first and submit the 8710 in IACRA afterwards? Or if you're submitting the 8710 in IACRA first but not completing some specific part of it until after the checkride?

E-Sign, using user name and password aka electronic signature.  And correct, checkride first then start the paperwork.  I suspect not, because that would give a 4th option - pretend it never happened, instead of Approval, Disapproval or Discontinuance.  Furthermore, if we wait to do the paperwork after the checkride a disapproved student might be able to claim that the checkride was invalid because they had not signed the paperwork until it was over.

Thanks, I reworded it a little to reflect what I think you mean and hopefully make it easier to get a good answer. If I got it wrong, you can always edit again or roll back my changes.

Thank you, spot on.

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How long would the engine of a Tecnam P92 Eaglet run for with the fuel selectors turned off?
I'm wondering about a scenario where a P92 has been flying for at least 30 minutes or so, at say 4,000 RPM and 2,000' AGL at standard temperature (I'm not sure if these details matter). With both selectors on, you would then shut off both fuel selector valves. With no fuel coming through the lines, how long would it take for the engine to starve and stop working?
Flight manual for reference: http://www.whangareiflyingclub.com/wp-content/uploads/2012/10/P92-Eaglet-LSA-flight-manual.pdf

If it isn't in the flight manual, what do you expect us to do?  Your not likely to get a good answer based on calculations taken from existing charts if that is your expectation.  If you are really curious, try it on the ground.  4000rpm ought to suck fuel roughly 4x faster than at idle.

yep that was my next step, but interested in what others like John K have come up with

No harm going to a safe altitude over an out of the way uncontrolled airport and trying it out!  Most airplanes will run on the ground for several minutes with the valve off, the perfect amount of time to give you an engine failure on take off when the left the selector off, but that's with idle operation most of that time (if you do a long taxi then runup, it'll probably quit during the runup).  Pretty sure any airplane won't run more than a minute or so on carb/gascolator fuel at cruise power.

It comes down to the fuel volume in the gascolator, lines, pumps, and float chambers of the Bing carburetors the Rotax 912 uses.  Without knowing the precise numbers, we can make an educated guess.
So say it's burning 17 l/hr at cruise, which is 283 CC/min.  I don't know the exact number, but say the Bing carb's float chamber holds 50 CC for a total of 100, and the gascolator/filter on the firewall holds another 100 cc of gas, maybe another 50 CCs on the lines between the gascolator and carbs, and another 50 CCs in the lines running to the shutoff valves. Add another 50 CCs for the pumps etc.
So say somewhere around 350 CCs of fuel is available once you shut off the supply, although the pumps won't be able to draw all of it due to the lack of venting at the opposite end at the valves, and it'll depend on the effectiveness of the fuel pump in creating a vacuum between the fuel and valve as it's drawn.  Maybe it'll be able to draw 300, which will run the engine for about a minute, maybe a minute and a half at the most.  Maybe quite a bit less.
So I would go with somewhere between 30 and 90 seconds.

I was pessimistic about a good answer coming out of this, but you have exceeded expectations!

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What does the SOG part of this METAR mean?

CWKX 231700Z AUTO 12001KT M05/M07 RMK AO1 SOG 32 6023 SLP123 T10471068

Does anyone know what “SOG 32 6023” means?
Also why is that type of metar have a bunny icon in it instead of the M or the classic circle?

Welcome to AviationStackExchange. SOG stands for Snow On the Ground. And, AO1 means that there is not an automated precipitation discriminator. I can not find any meaning for a “bunny icon”. Please include an image of the graphical METAR, please.

Welcome to aviation.SE! Could you tell us exactly which parts of the METAR you can’t read? It isn’t really clear if you’re asking about the whole thing or just some individual pieces.

Thank  for the answer, This part SOG 32 6023 i understand is snow on ground but those numbers what does mean? and the icon of the image

Thanks for clarifying, I edited your question to make it a little more specific. You might like to ask about the icon in a separate question, we usually prefer one point per question on this site. The [tour] might be useful if you’re new to the site.

Thank  you for helping me and i will ask the icon in other question so do i deleted that part of the question ?

The RMK section:
AO1 = an observation
taken by equipment without a precipitation type discriminator (rain/snow)
SOG 32 = snow on ground, thickness 32 cm
6023 = beats me, I'll keep looking...
SLP176 = sea level pressure: what the current barometric pressure at measurement location would be at sea level, 176 stands for 1017.6 hPa (10 is omitted as persistent)
T10471068 =temperature and dewpoint in two four digit groups: first 0 or 1 for above or below zero degrees, then three digits for tenths of Celcius grades, so in the case above temperature is -4.7 deg C, dewpoint is -6.8 deg C

In the US, the procode for a US METAR, “6”,  marks the start of where the METAR denotes the amount of precipitation in the previous 3 or 6 hours. In the US, the procode “6” is followed by four digits that represents precipitation observed in .01 inches. Six hours is reported on the 00, 06, 12, & 18 UTC hours. Three hours is reported on the 03, 09, 15, & 21 UTC hours. This is spelled out in FAA AIM section 7-1-12. Since the METAR in question is from Canada, I would hazard a guess that the three digits following “6” are in centimeters.


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Why can't we use bleed air instead of oxygen cylinders?
When cabin is depressurized due to some or the other reason then we usually have oxygen generators in airplane and they last for 12 minutes. It is very shot in usual terms but it is enough to descend a plane instead can we use bleed air so that it is long lasting?
Some expected counter questions and answers:
Q. What you will do for old people and small children?
A. For people aged above 60 and below 15 have to use oxygen generators however they are very less people in number as seen from normal aviation photos?
Q. What if they have problems like hypoxia?
A. Always while taking ticket they ask that do you have such kind of things
and they will be treated as above category?
Q. If it fails how can we do it ?
A. What you will do if passenger oxygen generator is failed?? That is my counter question however I am not telling to remove that oxygen generators fully but decrease capacity and decrease costs (note that same mask has both bleed air and oxygen from oxygen generator and this lining is light and cheap you can save more lives)?
Q. This will reduce safety
A. No never. Only it only decreases a bit of cost and it even can last longer and save lives more it is more safer reason: old people and children use only oxygen from generator and others use only bleed air
Q. If the air quality is bad?
A. The people use both oxygen generators and this bleed air and we can monitor air quality it is very very light and very very cheap like it is not even 10% cost of normal economy class ticket it is just bad to ask this. Even you can install multiple of them to monitor more clearly and and that will not cost much
Q. What if bleed air system fails?
A. My counter question what if the oxygen generator fails and what if normal bleed air system fails? It is like breaking the whole aircraft into 4 pieces in air. Of course there is oxygen cylinders in normal system if they don't descend it will become like Helios Airways flight 522

In order to breathe normally, you need a minimum oxygen partial pressure in the air. Normally, this is provided by pressurizing the cabin to higher pressures than the outside air with bleed air, which contains the same fraction of oxygen as the outside air, but due to the higher total pressure it will have a higher oxygen partial pressure. See also Why is the time of useful consciousness only seconds at high altitudes, when I can hold my breath much longer at ground level?
In case of a pressurization failure, the total pressure of the air in the cabin is reduced and the oxygen partial pressure will be insufficient for normal breathing. The oxygen masks solve this problem by providing (almost) pure oxygen, which results in a higher oxygen partial pressure at the same total pressure. If you would supply the oxygen masks with bleed air, this does not work because it would immediately lose pressure when exiting through the mask and only contain a fraction of oxygen.

What if the oxygen generators fail? Well, that can't really happen since that is not a single system. The bleed air supply is more likely to fail (although still very unlikely: you would need to have dual engine failure and APU failure (e.g. due to running out of fuel) or a dual pack failure). The oxygen system however uses one (or more) oxygen bottles for the flight crew (and their pressure is checked during the preflight) and chemical oxygen generators for the passengers. Each row has at least two of these generators, which are activated by pulling on one of the masks. They are all independent, so even if one of them fails, all others will still work and the cabin crew can get portable oxygen bottles to the row where one failed.
The Helios 522 accident report you linked (where the masks deployed automatically) actually has a good description of this system:

(AAIASB Final Report - Helios Flight 522)

What would stop you from designing a pressurised mask, rather than an unpressurised mask like the ones currently used?

@nick012000 Assuming you deal with the problems of sealing to the face, allowing breathing in and out, etc: consider that your lungs would have to withstand the pressure difference. You would inflate lungs like a ballon with far more force than a lung can withstand. https://www.mide.com/air-pressure-at-altitude-calculator Consider how many square inches of surface a lung has, and then something like a 10 PSI difference. Multiply the two. We're talking hundreds of pounds of force.

Because sometimes the cause of the depressurization is that there's a great big hole in the cabin.  E.g Aloha Airlines flight 243: https://en.wikipedia.org/wiki/Aloha_Airlines_Flight_243  Try to design a bleed air system that will pressurize that!
Now if you mean running bleed air through tubing to masks, what if your engines have failed?  Using an O2 system is redundant: it works even if the engines quit and there's a big hole.

why can't we use bleed air instead of oxygen cylinders?

Because if the cabin is depressurised, bleed air has already failed, and the oxigen tanks are there to provide a temporary help while the pilots bring the aircraft at a lower altitude.

Aircraft safety is built around the system of redundancy that has been established after many aviation accident case studies. In the case of cabin pressurization, bleed air is the primary system. A secondary system available to all passengers and crew must be available if bleed air is inoperable. You can not use bleed air as a backup to bleed air. And, you can not limit the backup to a limited amount of passengers.
As already stated, the chemical oxygen generators is a good compromise of reliability and weight. Their cost is about as negligible as designing a system to monitor he air quality and quantity received by each passenger. And, each row of passenger seats has two independent chemical oxygen generators to make their failure a statistical non-event.


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Is the capacity of an airspace sector related to the number of routes it crosses?
I've been searching for data about the capacity of sectors for my PhD research but somehow can't find any...the only thing I found is the routes it crosses ....can we conclude that it's the capacity or there is another factors involved. I would appreciate any help

Calculating the capacity of an ATC sector is a complex issue, you should not expect to find one single determining factor.
A few obvious factors come to mind:

Lateral extend of the sector (size)
Vertical extend of the sector
Whether or not surveillance is available (RADAR etc.)
The route structure, and whether traffic is forced to follow certain airways or not (free route airspace)
Military areas, their activation frequency and extend
Type of traffic in the sector
Uniformity of traffic in the sector
The actual traffic flow in the sector. Is it mainly an en-route sector where aircraft will enter and leave at the same level, or do a lot of aircraft need to climb/descend within the sector?
ATC staffing. Is the sector manned by a single controller, or by a pair (executive+planner)?
Radio coverage, and whether or not CPDLC is available
Weather - for example, is it an area with frequent thunderstorms?
Runway configurations of any airports covered by the sector, as well as possible noise restrictions

There is no formula that couples all these factors together. In the end, the capacity of an ATC sector is an estimate based on an evaluation of the above (and probably more) factors, experience and trial-and-error. In many cases, it will be an iterative process.

So we basically should focus on one factor to optimize ...thank you.

A fairly simple way to design sectors is to have one per VOR, with the boundaries halfway to the next VOR on each airway. This puts the VOR at the logical center of the sector with lots of space around it for the controller to resolve conflicts between aircraft crossing paths at that VOR.
If traffic levels do not justify having a separate controller for each sector, then multiple sectors may be worked by a single controller during low-traffic periods, or they may be merged into a single larger sector. I've read that the FAA targets 10-15 planes per controller.
Practical factors and experience, such as those listed by @J.Hougaard, would drive refining sector designs beyond what such a simple algorithmic model would indicate, but it gives a starting point—and a possible benchmark to measure how much the real world design varies from the theoretical one.

I appreciate your answer and the fact you stated a benchmark is interesting thank you


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What happens if I activate VFR flight plan later than filed ETD?
Let's say I filed a VFR flight plan with ETD (1500), en route (2 hours), ETA (1700)
If I depart and activate the plan at 1600, now the new ETA will be 1800 which is an hour later the the filed ETA. Is the ETA changed automatically on the system? or do I have to contact FSS and change to prevent initiation of the search and rescue?

When you call FSS to activate your flight plan you give them your time of departure. Unless your estimated time en route changes, the ETA will be amended correctly.

Are you asking about a specific country or jurisdiction?

Assuming that you've submitted a Delay Message in order to prevent your flightplan from being deleted you have to do nothing at all.
In your flightplan you only state your ETD (Expected Time of Departure) and ETE (Estimated Time Enroute). Your ETA (Estimated Time of Arrival) will be calculated and relayed to all stations in need of the ETA by AIS (Aeronautical Information Service).
To pick up your example:
Filed ETD: 1500
Filed ETE: 0200

AIS will relay ETA 1500+0200=1700 to your aerodrome of destination.
Let's say you've departed at 1554. Your aerodrome of departure will pass a departure message to AIS (so they know you are on your way) together with your ATD (actual time of departure) which would be 1554 in the example.
AIS will then calculate ATD+ETE => 1554+0200=1754 and relay the corrected ETA to your aerodrome of destination where the data in your flightstrip gets corrected.

In the U.S., a pilot can open (activate) a filed VFR flight plan beginning one hour before the filed departure time. The flight plan will expire out of the system two hours after the ETD. For an IFR flight plan, the period is two hours before to two hours after your ETD. If the flight plan expires, the pilot needs to refile.

You don't have to do anything. Your flightplan does not contain an ETA, and one is not calculated until you actually depart. You've only filed an estimated time of departure (ETD) and estimated elapsed time (EET).
When you depart and open your flightplan, a DEP message is sent to the reporting office responsible for alerting service at your destination containing your actual time of departure (ATD). They (their system) will then calculate an ETA as ATD+EET. This is the ETA that is used for the purpose of alerting service. The ETD of your flightplan is not used for anything related to alerting service. If you never depart and thus never open your flightplan, no DEP message is sent, and an ETA is never calculated - so alerting service is never activated. It's fairly common for VFR pilots to abandon their filed flightplan for some reason, e.g. file a flightplan and then never actually perform the flight. Such a flightplan is automatically deleted after a number of hours. It is never considered active unless you actually depart and have it opened.

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How to join a traffic pattern when passing overhead?
I can't seem to find a definite answer to the question on how to enter a traffic pattern when joining from the opposite end of the pattern. No matter who I ask, I'm getting a different answer every time, and they all seem convinced it's the only way to go.
So I'd like to ask a broader audience here.
Let's say runway 36 is in use, right traffic and you approach from the west. How do you enter the pattern?

I'm convinced "B" is the right way (safest at least) but recently I heard an ATC conversation where the pilot was instructed to cross overhead and join downwind. He used method "B" but the controller didn't like that because it took too much time.
What is the correct way? Is there a difference between controlled and uncontrolled airports? What about the procedure when approaching from other angles? And is there a difference between the U.S. and other parts of the world?
EDIT:
On March 13 2018, the FAA released the following Advisory Circular regarding this matter: AC-90-66B.

'A' would be my preferred method and the one I was always taught; it's the most efficient way to do it.  At a towered airport or beehive airport you're gonna catch a lot of flak from the controllers or other pilots for attempting 'B'.

Fly inverted along a track directly over the downwind leg but heading the opposite direction and a couple of thousand feet higher, and then when the coast is clear, pull some positive G's to position yourself in the downwind leg.

What is the correct way?
Is there a difference between controlled and uncontrolled airports? What about the procedure when approaching from other angles?

Well I'll address your second quesition first: There is ABSOLUTELY a difference between controlled and uncontrolled fields. At a controlled field you do what ATC tells you to do (and if there's any ambiguity you ask them to clarify what they want you to do).

At uncontrolled fields in the USA the standard way to enter the VFR traffic pattern is a 45-degree intercept to the downwind leg. The FAA's pronouncements on the issue can be found in AC 90-66A.
A brief and relevant excerpt:

a. Prior to entering the traffic pattern at an airport without an operating control tower, aircraft should avoid the flow of traffic until established on the entry leg. For example, wind and landing direction indicators can be checked while at an altitude above the traffic pattern. When the proper traffic pattern direction has been determined, the pilot should then proceed to a point well clear of the pattern before descending to the pattern altitude.
b, Arriving aircraft should be at the appropriate traffic pattern altitude before entering the traffic pattern. Entry to the downwind leg should be at a 45-degree angle abeam the midpoint of the runway.

(Emphasis Added)
You will find a rich variety of opinions (a polite way of saying "hidebound arguments") among pilots on the correctness of various other entries, but the 45-to-downwind is what's expected by most pilots in the US, and it is generally the safest way to enter the pattern (you will be where everyone else expects you to be, and you will have plenty of time to look at the pattern and figure out how you're going to sequence into it).
The 45-degree intercept assumes that you are on the side of the runway where you will be flying your downwind leg. If you are on the opposite side you need to position yourself to maneuver for a 45-degree intercept. I am personally a proponent of the technique illustrated below:

Cross midfield 500-1000 feet above pattern altitude.

At airports where there are multiple pattern altitudes specified, such as a "jet pattern" above the light aircraft pattern, cross at least 500 feet above the highest pattern altitude to ensure traffic separation.

Proceed away from the airport and clear of the traffic pattern before descending.
Make a descending turn to intercept the 45-degree entry

Ideally time your turn and descent such that you're at pattern altitude when you intercept the 45-degree track. If you expect to need more time to descend plan for a longer entry.

Enter the traffic pattern on a 45-degree-to-downwind.

Note that this is subtly different from your "Option B" (which has the descending turn leading directly into the downwind) in order to comply with the advisory circular's instruction that you should be "well clear of the pattern before descending".
I cribbed the pattern entry graphic from Alliance Airways, with some minor modifications, as it was the best illustration of the technique I could find.

My drawing should've looked like yours, because that's what I meant :-) So is my interpretation correct that it is best to cross above pattern altitude and make a 225 degree turn to enter downwind on the 45 coming from outside the pattern?

@PhilippeLeybaert That's how I was taught to do it, and it's the procedure that makes the most sense to me because it keeps you out of everyone's way. The entry is also very easy to fly: At standard rate the teardrop takes just over a minute, so if you are 500 feet above pattern altitude and you descend at ~500 feet per minute in the turn you should come out pretty close to pattern altitude on the 45 - just stop your descent when you hit your target altitude. If you cross 1000 feet above pattern altitude make the turn at half standard rate and it works out the same.

I've always done it like that in Europe but I was surprised to hear some people recommend other procedures here in the U.S. (like "A" in my example).

@PhilippeLeybaert At an uncontrolled airport, I don't think 'A' is really acceptable in the U.S. At a controlled airport, though, it's acceptable if it's what ATC tells you to do.

Just heard it again on LiveATC, talking to a pilot coming from the west: cross midfield and join right downwind runway 36. What does ATC want you to do?

@PhilippeLeybaert You're going to have to ask the tower what they expect here - we can speculate as to what their intent is all day long, but if you want to know what they expect the best way is to ask them to clarify.  To me as a pilot "cross midfield and join downwind" means "overfly the field and the downwind to join on a 45" so you avoid conflicting with traffic in the pattern, but at a controlled field they may want you to do something else to avoid conflicting with someone inbound.

@PhilippeLeybaert Also worth noting, when you're in controlled airspace it's expected that you'll comply with instructions by the shortest possible route/procedure - e.g. if you're flying 090 and you're told to turn to 180 you'll turn right (90 degrees) rather than left (270 degrees) - that's what controllers expect, so it would be totally legitimate for them to expect a pilot to fly "A" from your example with that logic as the basis even though it's not what I'd expect to do (or what I'd do at an uncontrolled field).

@voretaq7 Well, if I want to do it by the book, I should assume that the controller doesn't provide separation to VFR traffic in class D so it would be my own responsibility to avoid crashing into other aircraft. In that case I don't really care what is expected and choose to fly procedure B.

@PhilippeLeybaert If you are in controlled airspace and are told to "cross midfield and join right downwind runway 36", I would do it like your option A above. They didn't instruct you to overfly the pattern, fly a teardrop, do a 360 (or 270 in this case) or anything other than "join".  Much like joining final on an instrument approach.  I think that you will get scolded out quite often if you do option B after a controller tells you to join, because his spacing is based on the most expeditious method.  I've done this many many times and it's never created a problem as the other method has.

This topic can generate a lot of discussion as you've noted, but I think there are some generally accepted conclusions.
The simplest case is joining the pattern at a controlled airport because the procedure in any country is the same: follow ATC's instructions (and ask if they aren't clear!).
For unmanned airports the picture is less clear-cut, at least in the US where you can join the pattern however you like as long as it's safe. The FAA strongly recommends "joining on the 45" which is your option B:

When approaching an airport for landing, the traffic pattern should be
  entered at a 45° angle to the downwind leg, headed toward a point
  abeam of the midpoint of the runway to be used for landing. Arriving
  airplanes should be at the proper traffic pattern altitude before
  entering the pattern, and should stay clear of the traffic flow until
  established on the entry leg.

(Your option A looks somewhat risky, because to join right downwind at pattern altitude would mean busting through the left downwind side of the pattern, and it could also be in use by some traffic.)
But this joining method isn't an absolute requirement, and the traffic, terrain, local procedures etc. could require a different entry. Some pilots will even take a straight-in approach, especially in larger, faster aircraft and/or when there's little active traffic. (That's one point of possible controversy.)
Outside the US the procedures or regulations may be different. In South Africa I was taught a specific procedure for "joining overhead" at an unmanned field and (from memory) it was required by law. I've heard (and Wikipedia says) that the UK and some other Commonwealth countries also use it, but I don't have any direct knowledge of that. This is how it works and I was required to demonstrate it during my cross-country flight test:

You circle overhead the field at 2000agl for as long as it takes you to identify the active runway, wind direction, other traffic etc. Then you descend on the "dead side" - i.e. on the opposite side from traffic in the pattern - and cross the runway at the takeoff end at 1000agl before turning downwind.
The idea is that the procedure keeps you above pattern altitude for as long as possible to give you better visibility and more options if you need to break away. Descending on the dead side gives you good opportunities to spot other traffic already in the pattern as you descend, and crossing the departure end of the runway puts you well above departing traffic. This probably works best in high-wing aircraft.
Finally, I've also heard that military aircraft may use different procedures, at least at military airfields. SSumner mentioned in a comment below that the US Air Force uses either the 45-degree entry recommended by the FAA ("VFR entry") or a straight-in approach ("initial entry").

"The simplest case is joining the pattern at a controlled airport because the procedure in any country is the same: follow ATC's instructions" -> Is it that simple? As mentioned in my question, what if ATC says "cross overhead and join downwind"? Both A and B would comply with that instruction.

@PhilippeLeybaert I guess it's simple enough to ask ATC to clarify something if it isn't clear to you; that's standard procedure in any situation.

True, as always, but what is the expected procedure, especially in FAA-land? I have the impression that there are no clear rules, as opposed to Europe (where I learned to fly).

I learned the join on the 45.  It was worked so far.

This is an interesting (and seemingly dangerous) technique, I'm actually surprised it's required by regulation: Descending "on the dead side" and crossing over the runway as described potentially puts you in conflict with traffic executing a go-around, or a high-performance plane with a light load climbing like a homesick angel.

@PhilippeLeybaert Your impression is basically correct :-) At a controlled field in the US in your scenario I would expect (hope) to be instructed to join left base, or else be vectored to the pattern side of the field. If unmanned I would use B.

@voretaq7 Possibly, but in the context of short, often unpaved runways it's not so bad. You always have a good view and at least there's one and only one procedure to follow, so everyone is aware (or should be) of possible conflicts. A US pattern is rather unpredictable in comparison, although it's obviously dangerous to make too many assumptions in any pattern.

@Pondlife Yeah the big problem with VFR patterns at uncontrolled fields in the US is there's no regulatory standard - just a lot of "we'd really like it if you enter the pattern this way" pointers in various advisory circulars and handbooks. The net result is you have traffic joining in all sorts of "interesting" ways :-/

UK dead side joins are a throwback to be honest, I rarely use them because a) my home airfield is below a 1500ft class D, so overflying at 2000ft would take my ticket away and b) it takes longer than necessary, and c) I don't like descending directly over an airfield where other aircraft are taking off. I usually do a 45 degree join no matter what the CAA says.

@Pondlife - Air Force VFR patterns usually enter via 45 to downwind (often termed "VFR entry") or straight-in ("initial").

@SSumner Thanks for the information! I've added it to my answer

I think there must be a better phrase than "unmanned airports"-- and no, I'm not suggesting a related but gender-neutral phrase like "uncrewed" -- rather, "airports without control towers" or "untowered" is what is coming to mind.  Just a suggestion.

In the Navy, anytime we make a perpendicular entry to the pattern we turn directly downwind.  I'd have to agree with the controller, its a waste of precious time, especially during busy operations, to wait for someone to make a 360 and slam the pattern.  I can't see any useful safety benefit either, if you didn't see aircraft X as it was moving horizontally across your windscreen, what makes you think you'll see it any better by executing a 360?

The safety benefit of B is that you can cross the runway at a higher altitude. If you turn downwind directly, you would need to cross at pattern altitude.

How much higher do you plan on flying?  I can't see how it would make much of an appreciable difference.  Again, aircraft flying perpendicular to you offer the greatest visibility, if you miss it as you approach the pattern, I don't think a 360 is going to help you very much--even with the extra altitude.  Most importantly, if you are nervous or unsure, ask the controller, or elect to leave the pattern and fly out a few miles and then enter back in at a 45.

When an aircraft is turning, you can see it better because more of the plane is exposed to view. Making a 360 lengthens the amount of time you are in a turn, giving other pilots a chance to see you.

@SHAF According to the FAA Airplane handbook: "...aircraft should stay clear of the traffic flow until established on the entry leg". Using method A almost forces you to cross the upwind leg at pattern altitude, which violates this rule IMHO.

I understand your concern, but I feel like you're splitting hairs. You could easily, and safely, descend 500ft from the upwind side, through the turn, until established on downwind. For reference, I approach the pattern at 350+kts, 200ft above pattern altitude and break into the pattern for the carrier break. Oftentimes civilian airfields will also accommodate the overhead for us as well. The big picture item is listen, see, and avoid. Safety wise, all that matters is determining your proper interval and remaining predictable.

@PhilippeLeybaert Method  doesn't cross the upwind if you do it at mid-field....

@PhilippeLeybaert And it also says: "At airports with operating control towers, the tower operator may instruct pilots to enter the traffic pattern at any point or to make a straight-in approach without flying the usual rectangular pattern. Many other deviations are possible if the tower operator and the pilot work together in an effort to keep traffic moving smoothly. "  This is the norm at a controlled airport, and you don't need the same procedures as when you are on your own.

"350+kts, 200ft above pattern altitude" So.... you can pretty much disregard your advice for civilian operations? haha

I was instructed at an uncontrolled field in Canada some 15 years ago. For a right-hand circuit as shown, if arriving from what would be the west in your illustration, you cross mid field at pattern altitude and turn right to join the downwind. If arriving from the east, you should join the downwind leg at its upwind end. For a left-hand circuit, everything is reversed. In any case, you are typically making no more than a 90 degree turn to join the downwind and you don't cross it.
At a controlled airport, you will probably be cleared to join whichever leg is most closely aligned to your arrival heading; you might therefore fly straight in to the crosswind, downwind, base, or final leg.
This Wikipedia article includes procedures for a number of jurisdictions including the US and Canada (which is as I described above), and makes the point that things are done differently in different jurisdictions.

Yep, that's exactly what they teach in the UK. Trouble is, all the depictions of this "Standard Overhead Join" always show you coming in from the liveside, descending deadside, and then joining crosswind at circuit height. But what does it look like if you're arriving from the deadside? Easy answer would be to join straight into the crosswind leg at circuit height. An ATC may let you do that. However, at uncontrolled airfields, the expectation is that you spend enough time overhead the a/d in order to determine wind, rwy in use, etc. According to this, you'd have to come in on the upwind side above circuit height, loop around on the liveside above circuit height, cross the rwy again on the downwind side, still above circuit height, then head upwind on the deadside ("deadside descending"), drop to circuit height, and then join the circuit on the crosswind leg. That will give you a total of seven 90 degree turns before you land ...

Check this out from the NZ CAA:

This lesson covers vacating and joining at controlled and uncontrolled aerodromes. Elements may be combined with other training area exercises (refer CFI).
Where no other method is published, the standard overhead join procedure is the preferred method for joining the traffic circuit at an unattended aerodrome. It is used when the pilot-in-command needs to find out the runway in use, familiarise themselves with the aerodrome traffic and conditions, or when required by ATC.
The Standard Overhead Joining poster is a good resource for this
briefing.
Objectives
To vacate and join the circuit in accordance with applicable procedures.
To join an uncontrolled circuit in accordance with the standard overhead join procedure.
Considerations Discuss vacating the uncontrolled aerodrome first. As with a clearance, all of these methods are also available at controlled aerodromes.
Uncontrolled Aerodromes
Vacating
Leaving the circuit at an uncontrolled aerodrome is usually done from one of the circuit legs, climbing straight ahead on the runway heading to 1500 feet above aerodrome level, or from crosswind or downwind, or climbing to overhead – remembering that turns are always in the circuit direction.
Standard Overhead Join
Rule 91.223 Operating on and in the vicinity of an aerodrome requires the pilot to "…observe other aerodrome traffic for the purpose of avoiding collision, and, unless otherwise authorised or instructed by ATC, conform with or avoid the aerodrome traffic circuit formed by other aircraft." The standard overhead join procedure is a recommended means of complying with this rule, by being 500 feet above aerodrome traffic and then sequencing appropriately.
At an unattended aerodrome there will be no ATC instructions, and there may be no ATIS to forewarn the pilot of runway in use and wind conditions.
If the aerodrome is listed in AIP Vol 4, circuit direction will be shown on the aerodrome chart. This information and an estimate of the surface wind can provide a clue to the circuit direction.
Even if the runway in use is known, the pilot may elect to carry out the standard procedure if they are unfamiliar with the aerodrome layout and unsure of the location of other traffic.
The term 'overhead' is used because the aeroplane is flown over the aerodrome at a safe altitude above the circuit to look down and determine which runway is in use, or most suitable, and to sight any traffic.
It is important to realise that there may be traffic without a radio, NORDO (non-radio), and your only method of avoiding collision with them is to sight them and work out what they are doing from their position and movements.
Discuss the information found on the aerodrome chart that is applicable to the standard overhead join procedure. This includes; aerodrome elevation, runways available and their suitability, circuit directions, specific aerodrome instructions, the location of windsocks, and how to hold the chart to aid orientation. If the aerodrome is unfamiliar to the student, a study of the aerodrome chart should be carried out before flight.
Discuss the requirement to terminate the flight plan with ATC after landing at uncontrolled aerodromes.
Simulating a standard overhead join by 'walking and talking the pattern' beforehand can be a useful tool to prepare the student for this exercise.
Controlled Aerodromes
Vacating
When vacating a controlled aerodrome, all of the previously mentioned  options are available. In addition, a clearance to turn in the opposite direction to the published circuit may be given by ATC, or requested by the pilot. If a non-standard clearance is required, keep a good lookout and request it before takeoff.
Joining
When joining at a controlled aerodrome, the pilot-in-command has the option of requesting a standard overhead join. This is a good idea if the pilot is unfamiliar with the aerodrome layout, the active runway, or the position of the various circuit legs. ATC also has the option of instructing the pilot to carry out a standard overhead join.
However, the most common method of joining at a controlled aerodrome is to be cleared by ATC to join on the downwind, base, or final approach legs. Such a clearance may be for an aerodrome traffic circuit opposite to the published circuit for that runway; for example, "join right base" for a runway with a lefthand circuit.
Another possible clearance is to "cross overhead and join downwind". This is not a standard overhead join.
It is good aviation practice to establish the aircraft on an extension of the circuit leg to be joined, well before reaching the circuit area.
Where ATC is in attendance, but ATIS is not available, common practice is to request joining instructions. ATC will inform the pilot of the conditions and clear the pilot to join the circuit in the most appropriate way.
When ATIS information has been received before reaching the reporting point for circuit joining, the pilot should state or request from ATC the preferred method of joining.
A clearance to join downwind, base, or final does not absolve the pilot from giving way to other aircraft already established in the circuit.
Aerodrome Flight Information Service
Where an aerodrome flight information service (AFIS) is provided, joining and departing procedures specific to that aerodrome will apply. This may include the standard overhead rejoin.
When joining at an aerodrome with AFIS a radio call at 5–10 miles (or a position determined by the airspace) is required to state your position, altitude, intentions and POB. The AFIS will advise the ATIS, QNH and traffic information. The arriving aircraft then advises more specific intentions based on the information received.
Airmanship
Preparing the aeroplane for arrival involves the use of AIP Vol 4, the VNC (Visual Navigation Chart 1:250000 or 1:125000 if applicable) and joining checklists.
Revise the right-of-way rules and emphasise the requirement to make turns in the circuit direction.
If a downwind leg will not be flown, the aeroplane is prepared for landing before circuit entry by using joining and prelanding checks.
There is a tendency for the student to rely on radio calls during the overhead join, and to make too many. It is critical that a good lookout is carried out to identify all aircraft operating in the circuit, including those without radios.
The overhead joining procedure is used to determine the runway in use and the position of traffic in order to sequence accordingly. It does not presume a right of way over existing circuit activity. Orbiting overhead may be necessary until safe sequencing is available.
Aeroplane Management
Before entering the circuit, the aeroplane's speed will need to be reduced to below 120 knots, where applicable, as circuit speeds are normally restricted to this.
The landing light should be on.
Human Factors
As the student is orbiting overhead encourage them to get oriented by using the aerodrome chart and wind socks for indications of runway in use.
The limitations of vision are revised in relation to closure rates and objects that do not produce relative movement.
There is a lot of information to take in while approaching and orbiting overhead. Encourage the student to approach it systematically.
Air Exercise
Vacating
Discuss the way you would normally vacate your home aerodrome circuit, and the way you would vacate the other type of aerodrome.
Uncontrolled Aerodrome Joining
When joining at an unattended aerodrome, a radio call addressed to the circuit traffic is made between 5 and 10 NM from the aerodrome, stating your position, altitude, and intentions. The standard overhead join procedure is carried out in three main phases.
Standard Overhead Join
Approach
Approach the aerodrome to cross overhead at not less than 1500 feet above aerodrome level (refer landing chart) unless otherwise stated on the landing chart; for example, at Palmerston North 1500 feet amsl is used because of airspace above.
When calculating the altitude to join overhead at, round up – assuming there is no overlying airspace restriction. For example, if aerodrome elevation is 150 feet, rejoin altitude would be 1650 or 1700 feet not 1600 feet. The reason for rounding up is to maintain a 500-foot buffer over aircraft in the circuit, which may have rounded up the circuit altitude. At altitudes above 1500 feet it is harder to distinguish the windsocks, and more altitude will need to be lost on the descent. In addition, if all aircraft rejoin at the same altitude it should be easier to see each other.
The aeroplane should be positioned overhead and right of centreline, so that the student can look out of their window, down and across the whole aerodrome, observing traffic, windsocks and ground signals or markings.
Determine Runway in Use
Firstly the student must determine which runway is in use, as this sets which direction to make all turns, and the traffic and non-traffic sides.
Runway in use can be worked out by observing the windsocks, or other traffic already established in the circuit. If the runway in use cannot be determined quickly, make a turn to the left and continue to orbit left, until it is determined. If it is then found that a righthand circuit is in use, make all further turns to the right.
Be aware that many airfields have alternate circuit patterns for helicopter and glider traffic. If they are in use, joining aircraft must sequence into the circuit without causing conflict.
Both the traffic and non-traffic side must be identified to avoid descending onto aircraft already in the circuit. One method of identifying the traffic side is to have the student imagine they are lined up on the chosen runway ready for takeoff. If they were to takeoff which way would they turn at 500 feet – this establishes the traffic side.
Once the runway in use has been established and the circuit direction is confirmed (refer landing chart), a turn is made in the circuit direction to position the aeroplane on the non-traffic side. As aerodromes are potentially areas of high traffic density, use no more than a medium angle of bank. At this point it is good aviation practice to make a radio call advising traffic of the runway you are joining for.
Aircraft already in the circuit have right of way. This means that if aircraft in the circuit are using a runway considered unsuitable for your operation, the responsibility of avoiding conflict is on the joining aircraft, even if the runway in use is out of wind.
Regardless of which way the aeroplane is turning, the turn is continued until the centreline of the runway in use is crossed, and the aeroplane enters the non-traffic side.
Avoid giving too much attention to ground features during these phases, maintaining a lookout for other aircraft is more important as NORDO aircraft will not be heard and must be seen.
Descend to Circuit Height
When established on the non-traffic side descend to circuit altitude. A low rate of descent is preferred because of the potentially high traffic density around an aerodrome. Common practice is to use a cruise or powered descent.
The aeroplane must cross onto the traffic side of the active runway only when at circuit altitude. They should also track over the upwind threshold. Crossing the upwind threshold provides the longest possible downwind leg, while at the same time still providing maximum vertical separation from high-performance aeroplanes taking off.
As the downwind leg will be shorter than normal, the prelanding checks can be completed during the descent on the non-traffic side or on the crosswind leg (refer CFI).
On this crosswind leg, correct for drift, in order to track at right angles to the runway. A good lookout will need to be maintained for aircraft flying on the downwind leg. Do not turn in front of any such aircraft – always position behind.
The downwind radio call is made as soon as the aeroplane is established on the downwind leg, and the circuit is completed in the
normal manner.
Controlled Aerodrome Joining
The standard overhead join can be carried out at controlled aerodromes, with a clearance from ATC. More normally you will join via one of the circuit legs, usually downwind or via base leg.
Consult AIP Vol 4 for differences, particularly where there are neighbouring aerodromes.
Airborne Sequence
On the Ground
Make sure the student has all the necessary information to hand, and they have briefed themselves on the landing chart.
The Exercise
Vacate the circuit the way you are either cleared by ATC, or the way you discussed in the briefing.
They will have already seen, in previous lessons, how to vacate the circuit, but you may need to prompt some of the radio calls.
If you will be using your home aerodrome for the standard overhead joining procedure, head away in an unfamiliar direction, to add a little realism.
Begin the exercise between 10 and 5 NM from the aerodrome, over a local feature or VFR reporting point.
Talk through the standard overhead join procedure, and then have the student practise before any solo practise.
From the downwind leg, an approach and landing or go around can be carried out before vacating the circuit for student practice. If this is your home aerodrome, you may want to skip the landing, however, if it is an unfamiliar aerodrome landing practice would be beneficial.
After Flight
There probably won't have been time to cover every method of vacating and joining the circuit, so advise the student that you will take the opportunity in future lessons to practice these different methods.
Figure 1 - The Standard Overhead Join.

First off, there is a reason they are called "uncontrolled" airports.  Read the FAR's:  the guidance is just that, "guidance", there are very few "rules".... the local pattern police not withstanding.
While most of the above comments are technically correct and do not violate any rules, none of the comments address the jet operator who is faster and uses more real estate in a traffic pattern.  Jets are encouraged to use a 1500' pattern while non-jets use a 1000' pattern.  The result?  At some point, the jet is descending through the 1000' pattern into slower traffic.
See the problem?
Generally jets are equipped with TCAS and can see any aircraft that has an operating transponder.  The nightmare for the jet pilot entering an uncontrolled airport's traffic pattern at night and having a NORDO J-3 with a battery powered set of position lights (the J-3 is completely legal).  Can you imagine sailing down final at 140 Kts and trying to see the 45 KT J-3's dim, single tail light?
From my perspective, the safest thing a jet can do is enter a long (more than 2 miles) straight-in final, talk on the radio, watch for TCAS targets and be "in the pattern" for the least amount of time possible.
YMMV

The distinction for use of the 1500' pattern is generally turbine aircraft, not just jets. This would include turboprops such as the King Air or the DHC-8. Turbine aircraft also typically fly a wider patter, offering an opportunity to observe all lower/closer pattern legs until on final.

This does provide an answer to the question. It may be open to criticism (there may be little regulatory or advisory guidance suggesting that a straight in approach is preferred), but it certainly offers an answer that is valid for certain segments of the industry.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How are aircraft separated in a non-radar environment?
In remote areas that have no radar (oceanic airspace, etc.) how is aircraft separation maintained so that airplanes don't get too close?
What do areas that normally have radar do when there is a sudden radar outage?  There are airplanes going everywhere, not necessarily on standard airways, and a lot of them are being given radar vectors.  What happens when the screens go dark?

You are asking two very different, but related, questions:

How is separation provided in procedural airspace? (that is, airspace where no surveillance exists)
What are the emergency procedures when a surveillance system fails?

I will address them in that order.
There are three main types of separation: vertical separation, horizontal separation and separation in the vicinity of the aerodrome. I am going to assume you are not asking about separation in the vicinity of an aerodrome. We can divide horizontal separation further into three parts: lateral separation, longitudinal separation and radar separation. Obviously, radar separation does not apply in procedural airspace.
Common for all separation types, when used in procedural airspace, is that they are based on position reports made by the pilots. When we do not have transponders to broadcast the position and level of flights, the pilots have to manually tell us where they are.
Vertical separation is pretty straight forward. It is very easy to apply, and we often prefer vertical separation (even in radar airspace), simply because it is so easy. Vertical separation is obtained by requiring aircraft to operate at different levels expressed in terms of flight levels or altitudes. In most areas today, 1000 feet is the minimum vertical separation. 2000 feet is still used as minimum in some places. For more details, you should research Reduced Vertical Separation Minima (RVSM).
Lateral Separation is obtained by requiring aircraft to fly on different routes or in different geographical locations, as determined by visual observations, use of navaids or RNAV equipment. The idea is, if two aircraft are flying in two different places, then they won't collide.
One example of application of lateral separation is two aircraft flying on two different radials of the same VOR (radio beacon). If the aircraft are established on radials diverging by at least 15 degrees and at least one aircraft is 15 NM or more from the VOR, they are separated - per definition, regardless of the actual distance between them.
Another common example is approach procedures and different holding patterns. Certain procedures are designed in a way that ensures sufficient separation. For example, an NDB approach procedure might be separated from a nearby holding pattern. Again, we do not care about the actual distance between the aircraft - the two procedures are separated, by definition, if that is how they are designed.
Longitudinal Separation is used for aircraft at the same level, flying on the same, reciprocal or crossing tracks. Longitudinal separation is based on time or distance. For example, two aircraft following the same route are separated if there is 15 minutes between them. That is, if aircraft A reports overhead point XYZ at 09:12 and aircraft B reports overhead point XYZ at 09:30, they are separated.
Now, 15 minutes is a long time, and it can be reduced in certain circumstances, but these are the kind of rules we have to follow in procedural airspace (when vertical separation cannot be used). There might be a hundred miles between two aircraft with 14 minutes between, but they are not separated.
You can also base longitudinal separation on distance, if two aircraft are operating directly to or from the same DME (distance measuring equipment). In this case, the minimum separation is 20 NM (and can be reduced in some cases).
In addition to the two above rules (15 minutes or 20 NM), there are many, many rules regarding crossing tracks, reciprocal tracks, separation during level changes, separation between aircraft with different speeds, and so on. Common for all of them is that the pilots will have to report their position, and then the controller has to apply separation in accordance with the rules.
That sums up the separation methods and minima in procedural airspace. Now for your second question: what if radar airspace suddenly becomes procedural airspace as a result of a system failure?
Needless to say, this is extremely rare. Systems are very reliable, and there are backups, and backups of backups. That aside:
The first thing to realise is that this would most likely be considered an emergency situation, which means that normal rules and regulations do not apply. Many pilots don't realise this, but the word "mayday" is not actually reserved for pilot use only. You will likely hear a transmission from the ATC unit that sounds something like "Mayday, mayday, mayday, all stations, radar service terminated due to equipment failure, stop transmitting, standby for further instructions". At this point, you should make sure the volume of your TCAS is turned up...
However, it is important to remember that controllers are trained to handle situations like this. Local procedures are different, but the controllers will either attempt to establish procedural separation, or transfer traffic to adjacent units, if they have radar available. Even when the system fails, the controller will have some way (besides memory) to keep tracks of all the flights in their airspace. This could be physical flight strips, or an electronic equivalent.
The first thing we will do is probably try to establish lateral and/or vertical separation. The idea being, if all aircraft are flying in different locations, or at different levels, they will not collide. We are allowed to use half the normal minimum vertical separation (that is, 500 feet) in emergencies, and this essentially doubles the number of vertically separated levels available. Do not be surprised if you are instructed to fly at flight level 375.
What happens next depends entirely on the details of the outage. Traffic toward the affected areas will be kept on ground or rerouted, and adjacent ATC units with radar coverage will assist in evacuation of the airspace, which will most likely remain empty until the situation is solved. Controllers operating in radar airspace are probably not certified to provide procedural separation, so once the airspace is evacuated, it will remain empty until the system is back up - it cannot simply be changed to procedural airspace and normal operations resumed.

Wow, great answer, thank you!  I wasn't aware of the 500' separation option in emergencies.

For oceanic airspace, flights proceed along specified tracks and altitudes specified in their filed flight plan. From there, once they leave radar-controlled airspace, radio reports are made to ATC, who ensure that aircraft are appropriately separated if a flight needs to change its track or altitude. See the wiki article the North Atlantic tracks, which give a good overview on the track system. Specifically:

Prior to departure, airline flight dispatchers/flight operations officers will determine the best track based on destination, aircraft weight, aircraft type, prevailing winds and Air Traffic Control route charges. The aircraft will then contact the Oceanic Center controller before entering oceanic airspace and request the track giving the estimated time of arrival at the entry point. The Oceanic Controllers then calculate the required separation distances between aircraft and issue clearances to the pilots. It may be that the track is not available at that altitude or time so an alternate track or altitude will be assigned. Planes cannot change assigned course or altitude without permission.

and

Contingency plans exist within the North Atlantic Track system to account for any operational issues that occur. For example, if an aircraft can no longer maintain the speed or altitude it was assigned, the aircraft can move off the track route and fly parallel to its track, but well away from other aircraft. Also, pilots on North Atlantic Tracks are required to inform air traffic control of any deviations in altitude or speed necessitated by avoiding weather, such as thunderstorms or turbulence.

I'm going to leave the "what happens when things break" to someone else.

When the screens go dark ATC breaks out the flight progress strips (and possibly the shrimp boats or other airplane-substitutes to lay on a map & push around) and uses brain power to substitute for the computer and radar. Check out one of my favorite "Say Again?" columns over on AvWeb which talks a little bit about it (and you can find lots more if you browse the archives).

Broadly, there's a few implications to non-radar operations - both routine and "when stuff breaks" - most of which are concessions to safety. The one most pilots will feel, and the biggest safety item, is that "random RNAV routes can only be approved in a radar environment" (you'll find that gem in the AIM, but the translation is this: "GPS Direct? YOU NO CAN HAS!").
To facilitate separation in a non-radar environment the controllers are going to chuck you (and every other IFR flight) on airways, at standard IFR altitudes, quite probably with a specified airspeed, and they're going to route you navaid-to-navaid.
The idea here is simple: "You gotta put them together to keep them apart": By ensuring all flights are on known airways, at known altitudes, and at known speeds controllers have a good idea of who is where in their airspace.  They will supplement this mental picture by asking pilots to report crossing certain fixes (intersections/radials), so that they know exactly where that flight is when they make their report (which will help them account for winds aloft, and give them an idea of what the flight's ground speed is in addition to the airspeed they're asking them to fly).
All of the above provides IFR-to-IFR separation, which is what the controller is concerned with. IFR-to-VFR separation (which is normally supplemented by radar) becomes the responsibility of the pilots ("See and Avoid") - this means it's largely a factor of altitude (that 500-foot difference between IFR and VFR cruising altitudes and those cloud clearance requirements should start to seem pretty important right about now).

Any idea how they handle the transition from radar to non-radar during an outage?  I'm wondering how all of those airplanes already on "random RNAV routes" or receiving radar vectors can be transitioned safely back onto airways without two airplanes getting too close.  Good answer though and you get my upvote for now!

@lnafziger that part I'm not so sure about - Based on what I understand of the requirements, they'd have to get everyone onto airways as quickly as possible, but how to go about doing that when flights may be scattered all over your airspace on RNAV/direct routes is a problem I don't envy the controllers having. In "the old days" with everyone already on airways it would likely have been much easier. I suppose you can vacate an altitude and have them go direct to a convenient VOR at that altitude, which puts them on a known radial at a known altitude (& a known fix when they reach the VOR).

ATC aircraft separation is built upon the assumption of no radar by way position reports. At compulsory reporting points (black triangle on enroute charts), an IFR aircraft (unless radar identified) is required to report (see 5-3-2 of the AIM):

(a) Identification;
(b) Position;
(c) Time;
(d) Altitude or flight level (include actual altitude or flight level
  when operating on a clearance specifying VFR-on-top);
(e) Type of flight plan (not required in IFR position reports made
  directly to ARTCCs or approach control);
(f) ETA and name of next reporting point;
(g) The name only of the next succeeding reporting point along the
  route of flight; and
(h) Pertinent remarks.

ATC uses the position and ETA to keep aircraft separated. Most of the US is covered by radar, but remote parts of Alaska and oceanic still use position reports. If ATC radar is not functioning (rare), then the system reverts to position reports.
Moving forward, ADS-B allows aircraft to self broadcast its position. A network of ground and satellite ADS-B receivers can build up a picture of the traffic.


text:
Are VFR pilots required to avoid an isolated cumulus cloud like any other cloud?
When flying VFR, pilots are to stay free of clouds, which includes a minimum of 3 miles visibility, and 2,000 feet laterally.
Yet, I've seen some youtube videos where it appears the pilot gets too close to a "cotton ball" (a small, isolated cloud) simply because it is along his flight path.
If it is a very small, very isolated cloud, and you can see all the way around it, and have had good visibility on it for awhile as you approach, you know its not a "threat", right?
I suppose the FAA could violate you if you approach a cotton ball, but other than that, is there any reason to deviate and go slaloming around the sky just to keep distance from FEW clouds?

Related: How can I tell how far I am from a cloud?

Here is one such video.  It appears to me that he gets too close to some clouds, and this is a bit more than the "isolated cotton ball" scenario.  (maybe he's on an IFR plan... I don't know?)

If you can't see through it, then yes.  You need to avoid it because you don't actually know that there isn't a threat on the other side of it, headed towards you.

Your cotton balls are called "Cumulus humilis" a.k.a. fair weather cumulus.

"I suppose the FAA could violate you" - No. FAA staff are not allowed to violate(2) pilots. There are laws against that sort of thing.

@RedGrittyBrick "Sometimes it just feels that way."

It's entirely possible that the pilots in the youtube videos are on an IFR clearance.

"I suppose the FAA could violate you..." based on what evidence?

I suppose the FAA could violate you if you approach a cotton ball, but other than that, is there any reason to deviate and go slaloming around the sky just to keep distance from FEW clouds?

Aside from the legal reasons, keeping a healthy distance away from clouds is a Good Thing because other aircraft, birds, or alien spacecraft may pop out of them. Mid-Air collisions are no fun for anyone.
As far as going "slaloming around the sky", there's generally no reason to do that to avoid clouds:
If you're respecting the cloud clearance requirements (or at least the intent behind them) you shouldn't be getting close enough to clouds that you have to aggressively maneuver to avoid them - you've got plenty of distance to decide what you want to do.
Remember too that you've got 3 dimensions to work with: If we're only talking about a "puffy cotton-ball" or two you can always climb above or descend below them and stay on-course, or if they're all lined up inconveniently right along your flight path just offset your track a little bit (no reason to keep zig-zagging if a mile left or right of course keeps you clear).

That's not a good thing. I was planning to look for the giant's castle to get some gold coins.

I was wanting to get to Laputa!

Glider pilots call rows of cumulus clouds "cloud streets".  We fly directly underneath them to take advantage of the lift they generate; we can go long distances this way without needing to turn and without losing any altitude.  If you see these along your path, consider staying beneath them rather than alongside them to fly a little faster and maybe save some fuel.  http://www.soaringacademy.org/mtsafety/data/images/cloud-street.jpg

The rules are there for a reason, and yet they are broken regularly. Here in Europe, on a nice day for cross-country glider flying, every pilot climbs up until he/she almost touches the cloud. Thermals go up into big cumulus clouds (your puffy little ones are baby cumulus - just wait, and some of them will become really big), and the center is normally higher than the fringes (the updraft is below the center and continues into the cloud, the fringes show downdraft - the rising air has to come down somewhere).
Updrafts get stronger the higher they are, so the last meters below the cloud give you the best energy increase, and the pilots use the last circle not for climbing, but for speeding up. Then they fly straight out from the thermal, through the puffy fringes and out into the open sky at high speed (for a glider, that is).
It just happens. A friend once told me that he was below a cloud, and right next to him a turboprop airliner descended from the cloud. However, that must have been a bad airliner pilot, because flying through a cumulus like this incurs a lot of avoidable turbulence.

I doubt most airliners would deviate for mere Cu. And not all thermals are maximal right under the cloud base- although extra height is just about always welcome.

@yankeekilo: Most airliners operate under IFR, so the issue doesn't come up in the first place.

@Sean especially for mixed IFR/VFR traffic this is relevant, as the IFR traffic can drop out of the ceiling.


text:
Can pilots fly circling approaches at non-towered airports?
So the answer in my mind is "of course pilots can fly circling approaches at non-towered airports" (seriously, I could swear that I've done it before, but then again I can't think of any specific examples....).
That is, until I ran across this little tidbit in the Air Traffic Control Order while researching another question:

4-8-6. CIRCLING APPROACH
a. Circling approach instructions may only be given for aircraft landing at airports with operational control towers.

So then the question becomes, why do they have circling minimums at non-towered airports??

No tower here.
ATC can't clear me to circle.
Why do we have circling minimums??

It does sound rather vague. It seems that what it means to say is that ATC cannot prescribe one to circle unless at a towered field. The AIM says only 'This maneuver is made only after ATC authorization has been obtained and the pilot has established required visual reference to the airport' which seems to indicate that one can still request to circle-to-land

The way I understand that ATC order is that it is about "circling approach instructions" which is different than "(circling) approach clearance". They cannot tell you to enter a left downwind, do a right 360, or follow some other specific path to the runway. If they clear you to execute a circling approach to a given runway at a non-towered airport, then the path to the runway is for the pilot to determine, as necessary to separate yourself from other aircraft in the terminal area and land safely.

Well, the next paragraph says b. Include in the approach clearance instructions to circle to the runway in use if.....  This means that the circling instructions are part of the approach clearance, and a. specifically says that there must be an operational tower in order to issue them.  Apparently there is no circling approach clearance since they are just instructions appended to the normal approach clearance.

I think we are agreeing, that's why I put (circling) in parens. At non-towered airports, you pretty much just get an approach clearance. You can shoot that approach and then decide to circle and land at some different runway than the normal straight-in runway for that approach, if you break out and see a windsock that makes you think you should. The terminal area is yours to operate in and you'll be the only IFR in there.

Well, that's what I think too but am looking for a specific FAA reference which verifies your last comment.

I'm hunting too... will it be the ATC orders? the AIM? Instrument Flying Handbook? Parts 91/135/121? Let's play FAA bingo... will let you know if I find anything!

Haha, I've looked in a lot of that (not the Instrument Flying Handbook yet though).  Good luck in your hunt!

I agree -- the controller may only issue circling instructions when the approach terminates in a towered airport. In a non-towered airport below basic VFR minima, the controller may not issue circling instructions, and the pilot must consult the approach plate to see if he may circle, and then do so only within the constraints of the approach plate. The reason is fairly clear: only a towered airport can have approved weather reporting, and that approved weather determines whether the weather meets circling minimums.

(continued) without approved weather, approach control might issue a clearance to circle when the weather is below  circling minima, which the pilot is required (pace 91.123) to follow

"If they clear you to execute a circling approach to a given runway at a non-towered airport" — they will not. ATC issues the approach clearance only. How the pilot gets to a runway after breaking out is entirely their prerogative, as is the choice of runway in the first place. An approach clearance to an untowered airport is just that, a clearance for an approach to the airport, regardless of whether the approach is lined up with any given runway.

I've done quite a number of circling approaches at non-towered airports.  ATC clears one for the approach by type (VOR 4, VOR-A, GPS 32, etc).  There's no mention of how it is executed or terminates (other than missed approach instructions).  Want to fly the RNAV/GPS by the LNAV minimums, fine.  LPV, sure, go ahead, want to circle from 32 around to 4, they don't know how you're doing it.  They just know that the airspace around the airport is clear for you to execute and approach (and go missed).  Once you switch to CTAF, you're otherwise on your own.  There's nobody to give you a landing clearance either, but that doesn't mean you're not allowed to land!  :-)

Technically you are supposed to let them know that you are planning to execute circle-to-land. In reality, many circling minima are below 700 feet, i.e. in class G for most non-towered airports (not all of course), so ATC has no jurisdiction there anyway. I think the question was about the formal interpretation of a particular phrasing in the ATC order.

ATC-issued circling instructions are, by definition, instructions to land on a certain runway. At untowered airports ATC does not issue landing clearances, or indeed any instructions related to any runway, so obviously they cannot issue circling instructions. See the Pilot/Controller Glossary term and phraseology (emphasis in paragraph mine):

CIRCLE-TO-LAND MANEUVER- A maneuver initiated by the pilot to align the aircraft with a runway for landing when a straight-in landing from an instrument approach is not possible or is not desirable. At tower controlled airports, this maneuver is made only after ATC authorization has been obtained and the pilot has established required visual reference to the airport.
CIRCLE TO RUNWAY (RUNWAY NUMBER)- Used by ATC to inform the pilot that he/she must circle to land because the runway in use is other than the runway aligned with the instrument approach procedure.

There is no specific "runway in use" at untowered airports—all runways are equally "active" unless they are closed by the airport authority. The fact that ATC issues a specific approach clearance to an untowered airport does not relieve the pilot of their responsibility to maneuver as necessary to join any VFR traffic using the airport, and does not limit their ability to maneuver to a different runway better aligned with the wind; the pilot may:

land straight-in
circle-to-land on a different (or the same!) runway than the one aligned with the instrument approach
enter the VFR traffic pattern for any given runway, or
go around and fly the published missed approach.

ATC will neither approve nor disapprove such runway-related operations.

Thanks for the answer! You say that "ATC-issued circling instructions are, by definition, instructions to land on a certain runway." Do you have a reference and/or a quote that you can include in your answer with the actual definition that you are referring to?  This seems as if it would address the confusion which led to my original question!

@Lnafziger, edited.

As I understand 5-4-20 circling minima provide obstacle clearance within a lateral distance of the runway. In certain circumstances it may be necessary to overfly the landing environment to establish winds, etc. This should only be done when the pattern is in VFR conditions. 4-8-6 applies to clearances given not necessarily all safe operations.

ATC cannot give circling instructions at a non-towered airport. For instance, they can’t say “cleared RNAV 2 Circle 20”. They just clear you for the approach, and you decide if you want/need to circle. All runways are “active”.
When there is an (operating) tower, they decide which runways are “active” vs not, and Approach either clears you for an approach to a specific active runway (possibly including circling instructions) or, if you request an inactive runway for some reason, must coordinate with Tower to make sure that’s safe.
Either way, both Tower and Approach need to make sure your circle isn’t going to conflict with other departing or arriving IFR aircraft, especially if they’re using auto-release, which isn’t an issue at non-towered fields due to the one-in/one-out rule.

The definition of a circling approach read as follows:
"A circling approach is the visual phase of an instrument approach to bring an aircraft into position for landing on a runway which is not suitably located for a straight-in approach. (JAR-OPS 1.435 (a) (1))"
Now, if a non-towered airport has an instrument approach and at the end of that approach you see the runway then you circle around and land.  So the answer would be yes, you can circle to land at a non-towered airport.

This answer does not address the question: can a circling approach be flown at an uncontrolled field. Additionally, the second half of this answer is quite incorrect. At the decision height or missed approach point, if the runway environment is not in sight, a pilot must execute the missed approach procedure. Circling is only used when the runway is in sight, but the final approach course is not lined up with the landing runway.

If approaching an uncontrolled field at altitude a pilot can over fly the airport in order to determine wind direction and traffic in the pattern. The pilot can then begin circling while descending to pattern altitude and enter the pattern merging with other traffic. General aviation airports are not usually located where there are obstacles that have to be avoided during the approach unless there are mountains nearby that prevent a straight in final approach.

Many (but not all) non-towered airports with approaches in the USA have either AWOS or ASOS for weather information.  Obstacles are noted on the approach plate and may cause restrictions, such as "no circling west of the field".

" General aviation airports are not usually located where there are obstacles that have to be avoided during the approach unless there are mountains nearby that prevent a straight in final approach." is one of the most dangerous things I have heard uttered in a while. You must not fly into small airports out West much.

@BrianKnoblauch automated weather is not regulatory


text:
What are the differences between "ground stop", "ground delay", and "gate hold"?
I've heard several terms referring to air traffic control procedures that restrict movement for a certain period of time: ground stop, ground delay, and gate hold (plus others I'm sure I'm forgetting at the moment). How exactly does each of these procedures work? And do they affect arrivals, departures, or both?

The FAA Air Traffic Organization has issued a booklet titled "Traffic Flow Management in
the National Airspace System" which was specifically written for pilots and s flight operations personnel.  They cover Traffic Flow Management (TFM) in great detail (68 pages) but here is a small portion covering the most commonly used traffic management techniques.  Each section also includes an extensive "How does it work" sub-section which you can look at if you want more details.  I would highly recommend that any pilot read the entire booklet (while it is 63 pages, it is booklet format so it goes quickly)!
A GREAT website that the FAA has allows you to see a list of, and the current status of all of the following programs in one place:  http://www.fly.faa.gov/ois
Ground delay programs (GDP)

A Ground Delay Program (GDP) is a traffic management procedure  where
  aircraft are delayed at their departure airport in order to  manage
  demand and capacity at their arrival airport. Flights are  assigned
  departure times, which in turn regulates their arrival time  at the
  impacted airport.
GDP will normally be implemented at airports where capacity has  been
  reduced because of weather—such as low ceilings, thunderstorms  or
  wind—or when demand exceeds capacity for a sustained period.
GDPs are implemented to ensure the arrival demand at an airport  is
  kept at a manageable level to preclude extensive holding and to
  prevent aircraft from having to divert to other airports. They are
  also used in support of Severe Weather Avoidance Plan (SWAP).  (See
  section on SWAP.)

Airspace flow programs (AFP)

Airspace Flow Programs (AFP) were introduced in Summer 2006  and
  marked a significant new step in en route traffic management.  The
  principal goal for the initial deployment was to provide enhanced  en
  route traffic management during severe weather events. An AFP  is a
  traffic management process that identifies constraints in the  en
  route system, develops a real-time list of flights that are filed into
  the  constrained area, and distributes EDCTs to meter the demand
  through  the area.

Ground stops (GS)

A ground stop (GS) is a procedure requiring aircraft that meet
  specific criteria to remain on the ground. The GS may be airport
  specific, related to a geographical area, or equipment related.

Severe weather avoidance plan (SWAP)

SWAP is a formalized program that is developed for areas susceptible
  to disruption in air traffic flows caused by thunderstorms. Each air
  traffic facility may develop its own strategy for managing the severe
  weather event. Their plan then becomes part of the overall daily
  operations plan.

Special traffic management program (STMP)

A STMP is a long-range strategic initiative that is implemented when
  a location requires special handling to accommodate above-normal
  traffic demand, e.g., Masters golf tournament, NBAA Convention, etc.

Gate hold
Gate hold is not a traffic flow management technique, but is instead used by airports to keep you from starting engines and taxing just sit in line burning fuel when you can't takeoff due to a delay program.  It makes everybody's life better if you don't leave in the first place!

@Pondlife has good quotes and I'll supply some of the direct knowledge he requested.
Ground Stop
Your destination airport cannot accommodate arrivals -- at all.  In this case, they will disallow all planes destined for that airport to take off from wherever they are.
Some examples of what might cause ground stops are:

Weather impacting standard arrival routes, probably more than one.  Aircraft are already in holding patterns and they are running low on altitudes/fixes to put us, and/or those of us in holds are getting close to diverting for fuel.
Airport or runway closure.  If you can't land, you probably shouldn't take off.
All gates are occupied.  If planes cannot depart, there may be nowhere to park arrivals.

You'll note that many times you'll see ground stops described as "Tier 1", "Tier 2".  In short, the T1 ARTCCs are those that directly border with the ARTCC of the destination.  T2 ARTCCs are those bordering the T1 ARTCCs and so on.  Sometimes only T1 departure airports are affected by a ground stop, and sometimes it is everyone.
Ground stops are issued with update times, usually (in my experience) 45-60 minutes later.  They are subject to cancellation at any time, or extension with updated in another 60 minutes.
Ground Delays
These are like the Ground Stops but the airport knows it can accommodate you, just not exactly when you planned to be there.  The reason is most often weather at the destination airport.  Runway closures, accidents, construction, plowing/de-icing can all cause these delays also.
An example
In good weather, airport A may land 90 aircraft per hour.  The helpful airlines have scheduled 110 airplanes to land in one particular hour (no slots at this airport!).  Low clouds and high winds have moved in and now airport A can only land 40 airplanes per hour.  Now of the 110 airplanes, 70 of them will be delayed.  The airlines and ATC will work out which 70 planes those are, and which 40 planes will be allowed in that hour, and start issuing ground delay times.  Of course, these delays spill over into the next hour and cascade.
When a ground delay program is in effect, you will get a "Wheels up time" from ATC, which is when you have to be airborne.  Not sooner, and not much later (3-5 minutes usually).  Getting into your spot in the arrival flow depends on you taking off at this time.  In these scenarios ATC can figure out how/when to get you to your destination, if this isn't the case, that is usually when the Ground Stop program gets started.
Gate Hold Procedure
There is too much happening out on the taxiways, and no room for you.  Stay at the gate!
Reasons for gate holds:

De-icing is in effect and not enough room for everyone to line up at once.
The line to takeoff is too long
Your departure fixes are closed/saturated (This one happens a lot in the NYC Tracon)

In these cases, ATC will issue you a time to start and pushback via clearance delivery (at big airports) or ground control (smaller airports).  Sometimes ramp control can be helpful and help coordinate things... usually not.  When your time is up, you push and taxi and contact ground control.  Not any different than normal except you are given a specific time to do it.

At least for the US, the specific terms you mentioned are defined in the Pilot/Controller Glossary:

GROUND STOP (GS)- The GS is a process that requires aircraft that meet
a specific criteria to remain on the ground. The criteria may be
airport specific, airspace specific, or equipment specific; for
example, all departures to San Francisco, or all departures entering
Yorktown sector, or all Category I and II aircraft going to Charlotte.
GSs normally occur with little or no warning.
GROUND DELAY PROGRAM (GDP)- A traffic management process administered
by the ATCSCC; when aircraft are held on the ground. The purpose of
the program is to support the TM mission and limit airborne holding.
It is a flexible program and may be implemented in various forms
depending upon the needs of the AT system. Ground delay programs
provide for equitable assignment of delays to all system users.
GATE HOLD PROCEDURES- Procedures at selected airports to hold aircraft
at the gate or other ground location whenever departure delays exceed
or are anticipated to exceed 15 minutes. The sequence for departure
will be maintained in accordance with initial call-up unless modified
by flow control restrictions. Pilots should monitor the ground
control/clearance delivery frequency for engine start/taxi advisories
or new proposed start/taxi time if the delay changes.

Wikipedia has some more information:

Ground stop
Ground delay program

But I suspect that a really detailed explanation is probably only in the ATC manuals and procedures (and is probably far too long and complex to post here). I'm neither a commercial pilot nor a controller, though, so hopefully someone with more direct knowledge can comment.


text:
Can an air traffic controller refuse to give service to an aircraft based on moral grounds?
Today I learned about a Twitter account that tweets arrivals and departures to/from Geneva, of aircraft used by dictators.
I was wondering if an individual air traffic controller could refuse to give service to a given aircraft based on moral grounds, without legal consequences?
Maybe I'm a pacifist controller and I prefer to not handle a military aircraft sending troops somewhere. Or I don't like dictatorships and I don't want them in the country/airport where I operate.
Is there any legislation that could protect such objectors? To narrow a little the question, let's stick with Geneva in particular, or Europe in general.

I'm not sure if you would consider this an equivalency, but in the early 1990s during the buildup to the first Gulf war, Switzerland withheld overflight rights for U.S. military charters. As I remember, we had to avoid a few other states as well at first, but I can't remember which.

Interesting question, although I'm not sure that this is really specific to aviation. There's an employment law aspect (in what circumstances can employees legally refuse to do their job?) and a moral/philosophical one (is it better for society to provide the service for safety reasons, even if it enables undesirable outcomes?) and probably many others, but aviation seems to be a small part of this, IMO.

@Pondlife I was not sure whether to post it or not, finally I decided to do it. If it gets moved to other SO site, I won't object :)

"I was wondering if an ATCo could refuse to give service to a given aircraft based on moral grounds, without legal consequences?" Yes. They can resign. Unless, of course, they're actually in a dictatorship that doesn't allow that.

The way this CAN work is the state does not give the jet or the passenger diplomatic clearance to enter the country or the airspace. But it isn't an ATC function to keep dictators etc out of the country on their own.

Switzerland would be the country that is least likely to discriminate on political grounds. (@Terry's example is a different issue, as that concerns military operations.)

@reirab but if their contract requires them to give some notice period, resigning wouldn't relieve them of the duty to service this particular flight

@DanHulme Well, I meant if they don't want to handle such flights in the future. My point was if they don't want to treat all legal flights equally, they should get a different job.

The inherent problem with the premise of an individual ATC unilaterally refusing to provide service for a flight puts the other flights at risk as well as people on the ground.

@Terry : That seemed to have been a decision of the government, forcing all ATCo to comply, and not the decision of an individual ATCo.

on these grounds,there wont be many countries allowing air force one to land if Donald trump gets in!

No.
The purpose of air traffic control is to prevent collisions and expidite and maintain an orderly flow of air traffic. An air traffic controller has no say in who gets to fly and who doesn't. Refusing a certain flight would be something that needs to be done at a political level, and certainly far away from an operational level.
Also remember that, except in special cases, air traffic controllers have no way of knowing who is onboard a certain aircraft. Essentially, all we see is a flight ID and a route. Air traffic controllers are not in contact with passenger lists at any time.
Furthermore, air traffic controllers do not have any physical way of preventing flights from taking place. See this slightly related answer: Can a pilot take off at his own discretion?